1. Field of the Invention
The present invention relates to a process for purifying technical grade 1,1-bis(chlorophenyl)-2,2,2-trichloroethanol, commonly known as technical grade dicofol, for reducing its content of impurities related with DDT (DDT.R), to levels even below 0.1% by liquid-liquid extraction using highly selective solvents.
Dicofol is a widely used acaricide for agricultural purposes on cotton and fruit, citrus fruits in particular.
2. Reference to the Prior Art
The preparation of this product is described in U.S. Pat. No. 2,812,280 (Nov. 1957), U.S. Pat. No. 2,812,362 (Nov. 1957) and GB 831,421 (Mar. 1960).
The process initially used consisted of photochemical chlorination (scheme A) of technical grade DDT, a mixture of various products, the main components of which, however, are 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (p,p'-DDT, approx. 75% by weight) and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-2,2,2-trichloroethane (o,p'-DDT, approx. 20% by weight), by insertion of chlorine gas in the DDT at melting point. ##STR1## which led to the corresponding chlorine derivatives 1,1-bis-(4-chlorophenyl)-1,2,2,2-tetrachloroethane (p,p'-ClDDT) and 1-(4-chlorophenyl)-1-(2-chlorophenyl)-1,2,2,2,-tetrachloroethane (o,p'-ClDDT).
Because of steric hinderance, the chlorination of the isomer o,p'-DDT is incomplete, there remaining a residue of this substance which represented a content of about 2 to 7% by weight in the final chlorinated product.
The mixture resulting from the chlorination was subsequently submitted to hydrolysis in an acid medium (scheme B), consisting of heating to between 120.degree. and 150.degree. C. with an aqueous solution of sulphuric acid with an arylsulphonic acid. ##STR2##
The dicofol was extracted by dissolution in a solvent and decantation, the organic phase was washed with water and the dicofol was recovered by distillation of the solvent.
The o,p'-DDT was not affected by the hydrolysis and therefore the content thereof in the final technical grade dicofol was about 2 to 6%.
To reduce this content, the manufacturing process was modified by replacing the chlorination by substitution of the DDT with dehydrochlorination first (scheme C), with a strong alkali, to the corresponding ethylene derivatives, 1,1-bis-(4-chlorophenyl)-2,2-dichloroethylene, (p.p'-DDE) and 1-(4-chlorophenyl)-2,2-dichloroethylene (o,p'-DDE), ##STR3## followed by chlorination by addition (scheme D), which is also photochemical, to attain the same chlorinated derivative as in the previous case ##STR4## which was subjected to the same acid hydrolysis.
Nevertheless, the dicofol prepared by any of these processes contains a number of impurities related with DDT, called DDT.R, the nature of the following being known: ##STR5##
Other impurities of a known nature contained in dicofol and not considered to be DDT.R, are 4,4'-dichlorobenzophenone (p,p'-DCBF), 2,4'-dichlorpbenzophenone (o,p'-DCBF), 4,4'-dichlorobenzyl (p,p'-DCBZ) and 2,4'-dichlorobenzyl (o,p'-DCBZ). ##STR6##
As a consequence of the appearance of new specifications (Environmental Protection Agency, Federal Register, 29.05.1986), the DDT.R content of dicofol had to be reduced to 0.1% (by weight) as from 01.01.1989, which means that, for sale in the U.S. this product has to be purified.
U.S. Pat. No. 4,705,902 (November 1987) discloses a process for the purification of technical grade dicofol by liquid-liquid extraction (6,7,8,9) of the DDT.R down to a residual content below 0.1% (by weight). The only two Examples contained therein are: a simulation test of continuous extraction in a counterflowing column with four extraction stages and two recovery stages; and the other as a pilot assay in Karr column (25 mm diam). In both cases it is a question of extracting solutions of dicofol in methanol 90% in water with n-heptane, at an overall concentration lying between 20 and 30%, at atmoshperic pressure and room temperature (23.degree. to 35.degree. C.).